Pod sparse clones silently hide sibling-issue eval_results/ inputs (empty dir, no error)

Pod clones are sparse per-issue, so a driver reading a SIBLING issue's committed eval_results/issue_<M>/ silently sees an EMPTY directory instead of failing loud.

OBSERVED (#2569 leg 2, 2026-08-27). pod-2569-wa's clone had core.sparseCheckout=true with cone list: configs data docs eval_results/issue_2569 figures/issue_2569 scripts src tests scripts/issue2569_gateladder.py ladder defaults --config-dir eval_results/issue_1979/config and --race-dir eval_results/issue_1979/race — 55 files that ARE committed on the issue branch and DO appear in git ls-files (flagged S = skip-worktree), but were absent from the worktree. git pull reported "Already up to date" at the identical SHA, so the branch state looked correct while the inputs were missing.

WHY IT IS A WORKFLOW GAP, not an experiment bug:

• The diagnosis is non-obvious: SHA matches, git ls-files finds the files, git reports up-to-date. Nothing points at sparse-checkout. The only tell is the S flag from git ls-files -v.
• The repo ALREADY has this lesson for the LOCAL side — tests/sparse_cones.txt plus the CLAUDE.md rule that a new test hard-reading repo_root()/eval_results/issue_<M>/ must register its cone. The POD clone has no equivalent registry, so the same class recurs pod-side with no guard.
• It scales with cross-issue reuse, which the artifact-reuse rules actively encourage. Any plan whose driver reads a sibling's banked config hits it.

TWO CANDIDATE FIXES (not prejudging which): (a) At provision/dispatch, add a cone for every eval_results/issue_<M>/ path the plan references, the way new_worktree.sh pre-adds sparse_cones.txt. (b) Make the read fail loud: when a default-valued input dir is empty AND git ls-files reports tracked-but-skip-worktree entries under it, raise naming git sparse-checkout add <path> as the remedy. This is the cheaper, more general arm — it converts every future instance from a silent empty read into a one-line remedy.

WORKAROUND APPLIED THIS ROUND (unblocks #2569, fixes nothing durably): ssh pod-2569-wa 'cd /workspace/explore-persona-space && git sparse-checkout add eval_results/issue_1979' -> config 6 files, race 49 files materialized.

Filed from #2569; the ladder leg is proceeding on the workaround.